Solution to:
Eureka!

the bar of gold, which is kept floating at the start, but has sunk in the end.

The melting of ice does not change the water level, as explained in the solution to the previous question.

At the start, the upward force, needed to keep the bar of gold floating, equals the weight of the gold.
Therefore, the weight of the displaced water equals the weight of the gold.
In the end, the amount the displaced water equals the volume of the bar of gold.
Since the density of water is less than that of gold, the weight of displaced water is then less than the weight of the gold.

Conclusion: the water level will be lower when the ice has molten completely and the bar of gold has sunk to the bottom of the aquarium.